How to switch between fk and ik when they are separate armatures?
IK/FK switches are constructed using three groups of bones: FK chain, IK chain and a deforming chain.
The deforming chain bones have a set of constraints that make them copy the FK loc rot and scale, followed by a set of constraints that make them copy the loc rot and scale of the IK chain.
By animating the influence slider of this last set of copy IK constraints (generally using a single slider, and drivers to control all the constraints at once), you will switch between IK and FK, that can be in separate rigs without problems.
The premade sliders you see in the previous answer are related to Rigify rigs only, which builds for you the IK/FK set, adding some more convenient options, like a snap IK to FK and a snap FK to IK function.